The present disclosure is directed to a safety detonator intended for use in down hole apparatus, particularly for use in a perforating gun assembly.
A perforating gun assembly normally incorporates an elongate tubular sleeve or body which internally encloses multiple shaped charges. Upon detonation, the shaped changes form perforations extending outwardly radially of the well borehole and pass through the surrounding housing or assembly, and additionally form deep penetrating fluid flow passages through the surrounding casing, cement and into the adjacent formations. To assure proper detonation of the shaped charges, a detonator assembly is incorporated in the perforating gun assembly. The detonator assembly is connected to the surface via an electrical conductor, and when properly detonated, it provides detonation in a predetermined timed sequence to a detonation cord which connects with each of the shaped charges. The detonator assembly is therefore the key safety device in operation of the equipment.
Heretofore, detonator assemblies have been constructed with an electrically triggered detonator which is coupled through a passage or open space to a non-electric detonator adjacent to a detonating cord. On application of an electrical signal the electric detonator detonates, thereby, producing a shock wave or impulse which is transferred across the open space to the non-electric detonator. The non-electric detonator in turn is detonated, coupling the charge from the original electrical impulse into the detonating cord and to the shaped charges so that each charge of the perforating gun assembly is sequentially detonated. The detonator assembly has been intended as a safety device. There is a balance in the geometry of the detonating apparatus because the spacing between the electrically fired detonator and the non-electric detonator is crucial to safety.
The two critical dimensions of the spacing or passage, known in the industry as the "fire channel", coupling the electrically fired detonator to the non-electric detonator is the diameter (D) and the length (L). If D is too small, it acts as a choke and not enough force is transmitted through the fire channel to insure proper detonation of the non-electric detonator. If the distance L is too long, the same problem exists, i.e. not enough force is transmitted through the fire channel to insure proper detonation of the non-electric detonator. This often results in a low order detonation whether or not there is fluid in the fire channel. If the distance L is shortened to overcome the above described detonation problem, when dry, it increases the percentage of "fires" when the fire channel is filled with fluid, which is also undesirable.
Generally, the fire channel between the electrically fired detonator and the non-electric detonator is kept clear of well fluid. However, an opening is typically drilled in the detonator assembly which intentionally delivers well fluid into the fire channel. If the perforating gun assembly is exposed to well fluids, it is important that it not fire and fluid introduced in this region normally prevents firing. The length L must be sufficiently long that fluid in the tool dampens, even prevents transfer of the detonation shock wave. On the other hand, the components must be close enough to assure that the electrical impulse does in fact detonate the electrically fired detonator and make the necessary transfer to the non-electric detonator. Accordingly, the length L should not be too long or too short. If L is too long, misfiring will occur because the shock wave is attenuated as it travels through the long distance. If the length L is too short, then the safety system which responds to well fluids around the perforating gun assembly will not operate. As the length L is reduced, firing may still nevertheless occur because the well fluids do not totally prevent shock transfer from the electrically fired detonator to the non-electric detonator. Accordingly, this suggest that the length L be increased.
Control of the length L is thus difficult, being almost a balance of terror, where misfires occur because the shock wave does not get to the non-electrical detonator where L is too great, and unintended firings occur where L is too short and the perforating gun assembly is submerged in well fluids.
The present disclosure sets out a system which overcomes these risks and provides a much safer detonator assembly. The detonator assembly of the present disclosure avoids the dimensional sensitivity to the measure L as described above. Rather, the detonator assembly of the present disclosure couples the electrically fired detonator to the non-electric detonator through an open area which is in the form of a passage. The passage is somewhat short, sufficiently short to assure that coupling does occur so that transfer of the explosive shock wave assures detonation. The passage connecting the electrically fired detonator to the non-electric detonator is an open passage which is plugged by a solenoid operated plug. Thus detonation transfer into the passage is intentionally removed. Accordingly, the electrically fired detonator is not coupled with the non-electric detonator during transit, during arming of the device, during assemlby of the perforating gun, and at all other times. It is kept safe because there is isolation between the detonators.
The perforating gun assembly is a dangerous device to be handling. One of the dangers arises from stray electrical currents. The electrical currents typically arise in the context of handling such a device. It is normally loaded on a service vehicle such as a truck which carries a number of other devices and logging tools. It is not uncommon to load this device in the assembled state on a truck along with other logging devices. The truck normally is equipped with a reel or drum of electrical cable which is wrapped in a special fashion and which is otherwise described as an armored logging cable. The logging cable may support a great variety of electrical or nuclear logging devices which are carried on the same truck. All these devices connect with a variety of power supplies through the logging cable. The service vehicle normally connects the logging cable with one or more logging tools which respond to all types of electrical currents including high frequency AC, low frequency AC, and direct current, both positive and negative in polarity. The existence of electrical current generating equipment on such a truck runs the risk of creating stray currents, both in transit and at the site. Stray currents are a significant problem for perforating gun assemblies whether equipped with conventional detonators known heretofore or the high energy type detonators which are currently popular. High energy detonators require substantially more electrical power for operation. Accordingly, the truck mounted power supplies have large outputs so that high energy detonators can be triggered. The present apparatus takes advantage of a sequence of operations including polarity reversals to assure that the present device is fired intentionally, and does not fire in accidental circumstances. In other words, the device both in a stored situation or in a perforating gun assembly prior to intentional firing has a polarity sensitive circuit which assures that firing occurs only on the right voltage application to the device. Moreover, it includes means rejecting AC currents and hence does not fire when an AC current is applied to it.